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Mg-2.5wt%Y-4wt%Nd-0.5wt%Zr casting alloy was subjected to submerged friction stir processing (SFSP) with different rotation rates (ω) and travel speeds (υ). The influence of the ratio of ω/υ on the microstructure and mechanical properties of Mg-Y-Nd alloy was investigated in the present work by optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile test and hardness measurement. The results showed that the average grain sizes of SFSP samples were significantly refined compared with as-cast sample, and the coarse net-shaped Mg12Nd phases which located at grain boundaries in as-cast sample were changed into small particles. The combined effect of grain refinement and uniform particles distribution was responsible for the enhancement of mechanical properties. The relative optimal parameter of 600 rpm/60 mm·min-1 in this research obtained the finest grain size and the best mechanical properties, which were 1.1 μm for average grain size, 305 MPa for ultimate tensile strength and 22% for elongation, respectively.

Abstract: Friction stir processing (FSP) is a severe plastic deformation (SPD) method that has been applied to as-cast NiAl bronze (NAB) materials, which are widely used for marine components. The thermomechanical cycle of FSP results in homogenization and refinement, and the selective conversion of microstructures from a cast to a wrought condition. The physical metallurgy of NAB is complex and interpretation of the effects of FSP on microstructure has required detailed analysis by optical and electron microscopy methods. Annealing and isothermal hot rolling have been employed to confirm microstructure-based estimates of stir-zone peak temperatures. The variation of mechanical properties was assessed by use of miniature tensile samples and correlated with microstructure for samples from stir zones of single and multi-pass FSP. Exceptional improvement in strength – ductility combinations may be achieved by FSP of NAB materials.

Abstract: Grain refinement and high temperature deformation in two kinds of magnesium alloys
subjected to friction stir processing (FSP) have been investigated. One was a rolled sheet of
LA141Mg and another was a cast plate of AZ91Mg. FSP was developed by adapting the concepts of
friction stir welding to obtain a fine grain size in a stirred zone. Grain refinement was achieved by FSP
to give fine grain sizes of 11.4μm and 8.4μm for LA141 and AZ91 alloys, respectively. For LA141
alloy, the maximum stress of the FSPed sample was higher than that of the as-received one in the
range of 300K to 523K while the elongation to failure of the former was considerably smaller than
that of the latter. On the other hand, the elongation for the FSPed sample of AZ91Mg showed three
times larger elongation with a lower maximum stress than the as-received cast one at 523K and
2.8×10-3s-1. Further difference in high temperature deformation for both magnesium alloys was
discussed based on microstructural change and stress-strain curves.

Abstract: In this article, recent investigations on magnesium alloys by friction stir processing (FSP) are addressed. It indicates that remarkable grain refinement and breakup/dissolution of second-phase particles could be achieved simultaneously by FSP. High values of superplastic elongation were achieved in the FSP magnesium alloys at a wide range of strain rates and temperatures. The pinning of heat resistant particles on the grain boundaries in Mg-RE alloys stabilized the fine microstructure, leading to the occurrence of superplasticity at higher temperature and higher strain rate.

Abstract: Superplasticity in an AZ80 magnesium alloy subjected to friction stir processing (FSP) has been investigated. FSP was carried out at two traveling speeds of 150mm/min and 300mm/min for grain refinement. Optical microscopy on cross section to processing direction revealed obvious differences in size and feature between the stir zones at the two traveling speeds. The hardness of FSPed sample at the room temperature was about 30HV higher than that of as-received one. The maximum stress of the FSPed sample was reduced remarkably at lower strain rates compared with those of the as-received one at 573K and 673K. On the other hand, the elongation to failure of the FSPed sample showed ten to thirteen times larger than that of the as-received one at 573K and low strain rates. Further surface morphology near the fracture tip was observed by scanning electron microscopy to discuss deformation mechanism at high temperatures.

Abstract: Normal and submerged friction stir processing (SFSP) were conducted to AZ91 magnesium alloy plates with 6mm in thickness, and influence of processing speed (ν) on microstructures and mechanical properties of the experimental materials was investigated. The results revealed that fine and equiaxed grains were observed in the stirred zone (SZ). As the processing speed increased from 60mm/min to 150mm/min, the average grain size in the SZ of normal FSP material decreased. However, the grain size of the SFSP specimens first increased with the processing rate increasing from 60mm/min to 120mm/min, and then decreased when the processing rate increased to 150mm/min. Microstructure of the SFSP specimen was much finer compared with the normal FSP one, and the grain size of α-Mg was about 1.2µm when the processing speed was 60mm/min during SFSP. Because of much finer microstructure of SFSP, the microhardness, tensile strength and elongation were all improved. SEM fracture observation showed that fine dimples and tearing edges could be observed on SFSP specimen which showing good ductility. In addition, high temperature tensile tests showed that SFSP AZ91 alloys exhibited excellent superplasticity at high strain rate, with an elongation of 1202% at 623 K with a strain rate of 3x10-3s-1. The present study demonstrated that SFSP possesses great potential in preparing fine-grained materials.